It is known to utilize an electrical cable which supplies electrical energy to a downhole motor which drives a submersible pump in an oil and/or water well for pumping fluids. It has been proposed in U.S. Pat. Nos. 4,346,256 and 4,665,281 to utilize an electrical cable having a plurality of insulated conductors enclosed in an outer metallic tube.
One problem not covered is that the metallic tube wall thickness required to support the submersible motor and pumping unit weight in addition to the metal tube and its core weight is not practical using conventional metallurgy technology for use in well depths 8,000 to 12,000 feet deep. The problem lies in the materials used for the outer metallic tube. If a material is selected which has the tensile strength to support both the tube, its core, and the motor and pumping unit, higher strength materials must be used, but the higher strength materials tend to corrode faster in the well which leads to a reduced system life. On the other hand, materials which are corrosion-resistant, do not have the strength to support the metal tube, its core, and motor and pumping unit in well depths 8,000 to 12,000 feet deep.
The present invention provides a solution to this problem by reducing the tensile strength requirements of the metallic coil tube to withstand its own weight and the core weight only. The weight of the submersible pumping system is carried by a separate, retrievable support means which need not be corrosion-resistant. This system allows the use of a metal tubing with practical wall thicknesses using low alloy steels with improved corrosion resistance.
Another problem not considered by the prior art is the effect tensile loads and high temperatures will have on the relative motion of the inner electrical conductors to the outer metallic tube. Insulation materials used for the conductor insulation and jacket allow higher modulus materials, such as copper, to easily elongate and even yield the insulation. This condition is exacerbated over long lengths typically encountered in water and oilwells. The primary failure mechanism in electrical mechanical cables is conductor "z-kinking" whereby the conductors will twist radially leading to electrical failure. This is caused by higher coefficient of thermal expansion of conductors, such as copper or aluminum, versus the tensile member, such as steel, which leads to compressive loading of the conductors. This problem has been overcome by controlling the elongation of the two metal components of this system, the metallic tubing and the electrical conductors to allow optimum performance under tensile load and at elevated temperatures.